This invention relates to a variable capacitor adjustable by a linear motor. The linear motor is electrically adjustable and with circuit driver apparatus provides improved tuning and matching of NMR radio frequency probe coils.
An NMR spectrometer system is comprised of: a DC magnet which provides a stable, homogeneous static magnet field required for polarizing nuclear spins of a sample to be analyzed; a console containing an RF system which provides a suitable RF excitation source to the nuclear spins, and provides an amplifying and detection system for detecting and recording the NMR response signals from the nuclear spins; and a probe containing RF coils for coupling the RF excitation signals to the nuclear spins and for receiving response signals from the spins, and means for containing and positioned the sample within the probe coils to achieve optimum coupling between the sample spins and the RF probe coils.
For high resolution NMR studies the sample compound under investigation is usually dissolved in or mixed with a suitable solvent, is in liquid form and contained in a sample tube which is typically 5 mm in diameter. Solid samples may be a powder or crystal, and is some cases the sample may be contained in a magic angle spinning (MAS) probe for rapidly spinning the sample with the spinning axis tilted at an angle of approximately 54 degrees from the magnetic field axis. In either case the probe holds the sample tube and is positioned in the magnet so the sample is in the most homogeneous region of the magnetic field. The RF probe coil or coils for coupling the RF excitation to the sample and for detecting the NMR response signal must be tuned to the excitation frequencies and matched to the cable impedance leading to the preamplifier which may be located in the console or in the probe. The tuning and matching is typically done by variable capacitors that can be adjusted for optimum tuning and matching before running experiments with each sample.
Modern NMR spectrometer systems employ superconducting magnets typically consisting vertically mounted superconducting solenoid coils that are mounted in a Dewar structure with a central reentrant section extending up through the center of the superconducting solenoid coils. Typically the probe structure comprises a long cylindrical section that fits within the reentrant section of the magnet Dewar and a lower section that remains below the magnet Dewar that may contain a preamplifier and other parts. The sample and the transmit/receive RF probe coils are located in the cylindrical region of the probe. The probe is positioned in the magnet and Dewar structure so that the sample is centered on the center axis of the superconducting coils. This arrangement provides the most uniform magnetic field in the sample region. The space about the sample containing the RF coils and the tuning and matching capacitors is rather limited. Tuning and matching variable capacitors in this region have shafts extending to the lower region of the probe where they may be turned either manually or by motors located in this region or by more distant motors coupled by flexible cables.
A multi-frequency NMR probe has two or more RF probe coils with tune and match capacitors for each frequency. For example a triple resonance probe is capable of simultaneously operating at three different frequencies to excite three species of nuclear spins plus a “lock” signal. The “lock” signal typically is from deuterium nuclei in the solvent that may be deuterium oxide or deutero chloroform. To obtain optimum results with minimum excitation power, each of the four frequencies must be tuned and matched, requiring a total of eight variable capacitors. Because of limited space often compromises are made, and variable match capacitors may be provided for only one or two of the nuclei thereby reducing the variable capacitor count to 6 or less. In cryogenically cooled probes the RF probe coils are cooled to a low temperature. The coils may be either constructed of normal metals or high temperature superconducting materials. In these probes space is even more limited, with the further problem of heat transfer along the coupling shafts between the cold variable capacitors and the warm region at the bottom of the probe containing motors or knobs for manual adjustment. Heat transfer along these shafts puts an additional heat load upon the system used to provide the cooling.
Controlling the tune and match capacitors electrically provides the capability of remotely adjusting the tune and match capacitors thereby enabling the operator to remain at the console while tuning and matching the probe for optimum signal to noise ratio (S/N). To achieve it, the operator typically applies the desired excitation frequency and adjusts the tune and match capacitors to obtain a minimum of reflected power. Sometimes a small “dither” is applied to this frequency while the operator observes the reflected power from the probe. This enables the operator to readily determine whether the tune or match capacitor needs adjustment to further optimize the S/N.
It is also possible to use special software to control electrical signals applied to motors that are used to adjust the tune and match variable capacitors. Most prior art systems required a separate motor for each variable capacitor. As mentioned above, a shaft is run from the variable capacitor which is very close to the RF probe coil, to the motor at the bottom of the probe outside the magnet where space is limited. In all the prior art systems using superconducting magnets, one or more rotatable shafts were required to transmit the rotary motion of the motor located in the base of the probe to the sample region where the probe coils and tune and match capacitors are located. In cryogenic probes, the probe coils and tune and match capacitors are cooled to a low temperature. Problems with these systems include heat loss arising from heat being conducted up the rotatable shaft from the motor region which is near room temperature to the sample region which typically is at a very low temperature, usually at 25 K or less. To avoid a temperature rise due to this heat loss, additional cooling power is required. An another problem is maintaining alignment of the various parts in the cooled region. Forces are generated and transmitted along the rotatable shafts during the cooling phase causing misalignment of the NMR probe coils with each other and with the external magnet and gradient coils.